SX-RK3 Sturmey Archer’s 3 Speed IGH For Fatties : “I’ve Got The Power” *dum* *dum* *da* *da* *dum*

I’ve been pretty bullish on the Nexus-3 IGH for fat bike builds the last couple of months. The Nexus-3 IGH only adds a little over a pound to the weight of your ebike, but it drastically increases the usable range of your BBS02 or BBSHD motor over a single speed setup. It seems to be able to easily take 1500W of power, even when shifted under severe loads as well as being able to handle higher speeds of over 25mph without a problem. The only disadvantage of the Nexus 3 is that it will only fit on 135mm dropouts (like my Sunkiss and Duh Banana Bike builds) which are as rare in the fat bike world as an albino alligator. Luckily, Sturmey Archer built and is selling a 3-speed IGH that is specially designed for fat bikes called the SX-RK3 (their website information page is

Luckily, Sturmey Archer built and is selling a 3-speed IGH that is specially designed for fat bikes called the SX-RK3 (their website information page is here) which fits perfectly in 170mm rear dropouts and only weighs in at 1400g without the cog or shifting mechanism. This means that you can install a 3-speed IGH on most 4″ tire fat bikes on the market and only add a little over 2 lbs to the total weight of your fat bike (most decent rear hubs are about a pound). The SX-RK3 hub is available from bikeparts.com for about $95 here but you will also need some other mounting hardware and a shifter to make it work.

A sweet addition to any fatbike that will resist snow and sand but still give you plenty of power and speed

A sweet addition to any 170mm fat bike that will resist snow and sand but still give you plenty of power and speed

I tested a Sturmey Archer 3 speed IGH on my Burning Chrome build which is essentially a polished Soul Stomper with a 3000W Cyclone kit installed on it (installation instructions for the Cyclone are located here). I did not have to lace up the IGH to a fatbike rim which was a nice change as it came installed on the bike already. Getting the IGH set properly was a little tricky but there is an easy video on youtube located here that Jim who owns Soul Bikes put together. Like most bicycle videos on the internet (Bruno excluded) this video is terribly boring and SPOILER ALERT Darth Vader is Luke’s father, I know big surprise. What you already knew that? Worst kept secret in the galaxy.

Although the Sturmey Archer IGH with lots of gears seem to have mixed reviews, for the most part the 3-speed SA IGH has been quite bulletproof through the years. I’ve read lots of stuff on the internet from people who had the SA 8-speed IGH units self-destruct on them with normal biking activity and any powered e-bike is encouraged to use an Alfine 8-speed IGH (~$200 street price) if you want more than 3 gears. There is also the ridiculously expensive Rohloff Speedhub XL 14 gear IGH for fat bikes, but that runs around $1400 which is more than I pay to build most of my e-bikes with a battery and motor. I have never tested the Alfine or the Rohloff so I can’t speak to either one, but buy either one and ship it to me and I’ll beat the crap out of it and then send it back once I figure out how much power it will take to destroy it. See, this is why we can’t have nice things.

This shows everything you need to make the SA SX-RK3 work properly with a grip-sh!t shifter

This shows everything you need to make the SA SX-RK3 work properly with their grip-sh!t shifter

The SA SX-RK3 I tested had the grip shifter which seemed to work fine, unless you are using a right-hand throttle. I was using a right-hand throttle so I swapped the grip shifter to the left-hand side and all the numbers were upside down. This was done intentionally to confuse potential bike thieves. There is also a trigger shifter which I have never used but many people have recommended that I try that is sold by SA and I’m told that it works properly with the Nexus-3 IGH as well. The SA uses an industry-standard 3-speed IGH cog and can take a cog down to 13T according to their spec sheet.

sa cropped

The RB3 on the right is the Servo Brake option which you should never-ever use. An e-bike would destroy it handily, then you would crash and die.

As you can see from the chart above when the SA IGH is in the center gear it is a 1:1 gearing ratio. This might have a lot to do with why the SA hubs are so indestructible, other IGH that have the LOWEST gear set to a 1:1 ratio tend to be more fragile because when you end up in the higher gears there are a lot more of them and they have to be smaller. Looking at the planetary gearing in the SA 3 speed IGH the gears are big and have lots of teeth. I intentionally tried to shift under load at 2000W (which you should never ever do) and it was silky smooth with no issues at all. The Nexus 3 in comparison make a lot of strange noises when you try to shift it under load that make it sound like if you keep doing it you will be sorry. There are lots of people on the internet that claim to have run 4000W through this hub for many miles with not issues, but they never shifted under load.

Here you can see the SA 3 speed IGH internals, the massive freewheel in the center is what makes the loud clicking noises

Here you can see the SA 3 speed IGH internals, the massive freewheel in the center at the bottom of the gearing system is what makes the loud clicking noises

The only complaint I had about the SA 3 speed IGH was the continuous clicking from the freewheel when the bike was being pedaled normally. Although everyone who rides a bike (except those hipster fixies) is used to the sound of clicking free wheels, but not when we are pedaling down the road, only when coasting. I have to admit that the constant clicking noise that emanated from the SA hub was pretty annoying. I’ve heard that this clicking goes away after a while, but mine hasn’t yet. Although it’s not that audible, it is constant, and it is annoying (kind of like my wife’s cat).

The Sturmey Archer 3 speed IGH works awesome even at high power under load

Most SX-RK3 I’ve seen online are silver but this one shipped matte black which I actually prefer, shown with an 18T cog this bike ended up with a 44T chainring and a 15T cog for high-speed commuting

Gearing

Figuring out the right gearing for the SA is tricky. Your best bet is to get the cog that you think you are going to need then ride the bike around and see if you need a bigger or smaller chain wheel or cog. The cogs for the 3-speed IGH are really cheap (I got my 15T for about $6 shipped on ebay) so it’s usually easier and cheaper to swap the cogs than the primary chain wheel. For most builds the rule of thumb is that if you want to use if for high-speed road riding you want a much smaller cog in the rear (around a 14T or so). If you are using it for trail riding then you will want to buy the biggest cog you can get for this IGH which is a 22T. There is a guy who runs the Bikesmith who sells aftermarket 26T cogs for a whopping $27 located here which would probably work best if you were running a 42T chainring (like the Lekkie or the Luna Eclipse rings) and wanted to have any prayer of having decent torque when trail riding. Adding a larger cog will cut into your top speed, but will give you way more power.

If you don't want a grip sh!t shifter then this is your only other option.

If you don’t want a grip shifter then this is your only other option. $33 from amazon here. Normal shifters will not work with an IGH.

In short, the Sturmey Archer 3 speed IGH seems to be a realistic, viable option for 170mm rear dropouts that will work with most mid drives up to 3000W without issues. If you can get past the clicking and the cost and are willing to lace up your own wheel then it might be a great option for you. The SA 3 speed IGH is only available with 36 spokes and almost every fat bike rim in existence is 32 Hole. The only 36H fat bike rim I have found that will work with this IGH is the Weinman HL80 (available here) which is quite a heavy rim without punchouts and runs for around $50 online. The SA 3 speed IGH will only work with dropouts that accept threaded or quick release axles and will NOT work with through hole axles. Be aware that with a single speed IGH you will want either horizontal dropouts or a chain tensioner as otherwise it will be impossible to get the chain tension right. A loose chain at high power makes for all kinds of problems.

Destroy all monsters.

Ride On.

 

37 thoughts on “SX-RK3 Sturmey Archer’s 3 Speed IGH For Fatties : “I’ve Got The Power” *dum* *dum* *da* *da* *dum*

  1. Great write up, Karl. If you still have the hub apart, would you please measure the length of the gears? I am wondering if the gears are the same as the common 135mm drop out version, or if maybe the extra length of the shell allowed them to use longer gears (which would be stronger).

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  3. Thanks for the excellent article Karl. I was looking for an IGH for my fat bike cruiser as I had seen that Sikk Inc sell the same cruiser as mine with the Sturmey Archer gears, having recently bought the single speed version of the cruiser from a UK seller. Now I know what to buy, thanks for the great articles, informative as ever, keep up the great work…many thanks.

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  6. I really thought the sxrk3 was the ideal solution to my bbshd gear problems. Damn thing died on me yesterday though. Hoping its repairable as it was great really smooth and no crunching. Gone back to derailleur now and missing the SA already.

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  7. Karl did your axle flats fit tight into your drop out openings? My drop out openings were about 2mm larger than the axle flat dimensions. I think this will be a problem for a lot of fat bikes and Sturmey does not seem to have an off the self solution for this. I got mine to work but lots of tinkering to get it to work for my bike.

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    • Hi Karl, Wayne, have one of these but like Wayne the 8mm flat of the axle on this hub is smaller than my 10mm wide horizontal drop out…the bike weight seems to rest on the top side of the axle, and the anti rotation washer tabs kind of fit in, but it doesn’t look very secure with a 2mm gap at the bottom! Can I ask what either of you did to get a snug fit for the 8mm axle in a (guessing 10mm) dropout? thanks!

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  9. Looking for an affordable frame with 170mm rear spacing and horizontal dropouts to put this on. Any ideas? Or are folks just using chain tensioners?

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    • I have not run across an affordable fatty frame with horizontal dropouts and 170mm spacing, although I’m sure they exist. You can use any derailleur as a chain tensioner, just get a really long adjustment screw for it. They will have them at any bike shop. The short cage derailleurs work best.

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  10. how did this Hub hold up after a year at 3000 Watts.? Did you snap the axle? can you still recommend it? as above where can I get a black one?maybe sikk bikes would sale one?

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  11. Thanks for these articles, I’m interested in building a commuter from my wabi classic (single speed). Currently only have 120 mm dropout spacing… Contemplating cost effectiveness of IGH wheel build + bbshd. Or perhaps just the bbshd on single speed alone??

    I have both steep and long hills in my 12 mile one way commutes. Does the bbshd need gearing to be reliable/safe support going up hills?

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    • Are you sure it’s 120mm? That is a weird hub spacing. You can probably squeeze a Nexus 3 135mm hub in there without damaging the frame.

      Mid drive is the only way to go for steep long hills. The BBS02 or the BBSHD will work. The BBS02 will be a bit cheaper and have a cleaner chainline.

      I’ve done single speeds, but you will be much happier with a 3 speed IGH. It is the perfect system to pair with a mid drive.

      Karl

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      • Thanks for your reply, I am leaning on building an IGH now… I was doing more research and looks like the two IGH options for 120mm OLD are Sturmey Archer 3 speed and 8 speed. I decided against altering the dropouts for a larger hub in case I want to switch to my SS and tool around.

        I know that the 8 spd is direct drive on it’s lowest gear. Does that mean that it would be the sturdiest gear to use for motor assisted hill climbs? What sprocket/chainring combo would you recommend?

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  12. I’m working on a build with a Cyclone Mini on a Soul STOMPER XTR. it seam’s as though there is a huge amount of gap before the hub engages. (maybe as much as 25-35 degrees of crank rotation) I do think that the motor needs some more mounting improvements for rigidity but my initial reaction to my maiden trip around the block was “damn that can’t be right “. I suppose if I always pedal as I apply power I can take up that slack before the cyclone rips but still don’t seem right.

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  13. I see that SIKK includes black ones on some of their bikes, don’t know if they will sell the unit separately in black or not. Just discovered this IGH for a build I am considering, belated thanks 1.5 years later Karl for the great write-up.

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